Bus switches are widely used for networking applications. Metal-oxide-semiconductor (MOS) bus switches feature low on resistance, reducing delay through the switch. See for example "Parallel Micro-Relay Bus Switch for Computer Network Communication with Reduced Crosstalk and Low On-Resistance using Charge Pumps", U.S. Pat. No. 5,808,502, and "Bus Switch Having Both P- and N-Channel Transistors for Constant Impedance Using Isolation Circuit for Live-Insertion when Powered Down" U.S. Ser. No. 09/004,929. The source and drain nodes of the transistor connect to the busses while the gate is controlled by a bus-connecting enable signal.
FIG. 1 shows a typical application of a bus switch. Local bus signals 18 (bus A) is connected to CPU 12, memory 14, and Application-Specific Integrated Circuit (ASIC) 10. Hot-plug bus signals 19 (bus B) is a hot-plugable expansion bus such as for PC or PCI cards. Switch network 16 connects address, data, and control lines from bus signals 18 to bus signals 19 using MOS transistors. Once transistor is used for each bus signal.
When a device is plugged into bus signals 19, it may be desired to isolate bus signals 19 from local bus signals 18. Noise caused by the plugging operation can then be isolated to bus signals 19, allowing local bus signals 18 to operate unhindered. Switch network 16 can isolate bus signals 19 from local bus signals 18 by applying a low voltage to n-channel transistors in switch network 16.
FIG. 2 illustrates a single bit of a bus switch. A signal from local bus 18 is connected to a corresponding signal on bus 19 by n-channel bus switch transistor 22. When a high voltage is applied to the gate of bus switch transistor 22, it allows current to flow from drain to source, connecting a signal in bus 18 to bus 19.
P-channel pullup transistor 24 connects the output on bus 19 to the power supply when bus switch transistor 22 is non-conducting. Pullup transistor 24 prevents output bus 19 from floating when bus switch transistor 22 isolates bus signals 18, 19. When the gates of bus switch transistor 22 and pullup transistor 24 are driven with full-rail voltages, only one of transistors 22, 24 is on at any time. The bus switch acts as an open circuit when isolating bus signals 18, 19.
Undershoot Problem--FIG. 3
FIG. 3 illustrates an undershoot problem with the bus switch of FIG. 2. When the bus switch is disabled by driving a ground voltage to the gate of the n-channel bus-switch transistor and the p-channel pullup transistor, the output to bus signal 19 should be isolated from voltage changes at the input, bus signal 18. The quality of the signal waveforms on local bus signal 18 is not always well controlled. Sometime large voltage spikes below ground (undershoots) occur, especially on the high-to-low transitions from high-current drivers on local bus signal 18.
When the bus-switch input from bus signal 18 goes below ground, a positive gate-to-source voltage develops on bus-switch transistor since its gate is at ground. A conducting channel forms below the gate. When the undershoot is greater than a volt, this gate-to-source voltage exceeds the n-channel threshold voltage, turning on the n-channel bus switch transistor. Some current is conducted through the channel of the bus-switch transistor even though its gate may be kept at ground. The result is that the voltage is disturbed on the drain of the bus-switch transistor, and the output to bus 19.
When the source of the n-channel bus-switch transistor goes negative during the undershoot, the base-emitter junction of the parasitic lateral NPN transistor is forward biased, coupling more current to the output through the p-type substrate.
The result of the undershoot is that the output connects to the input for a short period of time, the duration of the undershoot. The voltage on the drain of the bus-switch transistor can quickly fall from the power supply (Vcc) to ground and even below ground should the undershoot last for more than a few nanoseconds. Thus FIG. 3 shows the undershoots on the input bus coupled to the output, producing severe voltage disturbances on the isolated bus.
One solution is to use a diode to clamp the input to ground. FIG. 4 shows a bus-switch protected from undershoots by a Schottky diode. Input bus signal 18 is coupled to Schottky diode 28, which turns on and conducts current to ground when the input on bus signal 18 falls below ground.
While Schottky diodes were common for bipolar processes, most standard CMOS processes do not include Schottky diodes. Extra expense and process complexity is required to include Schottky diode 28. Thus Schottky diode 28 is undesirable for integrated circuits (ICs). Adding an external Schottky diode 28 also adds expense and consumes board space.
Another solution is to add a series resistor to dampen the ringing that causes undershoot. FIG. 5 highlights using a series resistor to reduce undershoot. Series resistor 29 dampens signals from bus signal 18, preventing or reducing undershoot seen at the source of bus-switch transistor 22. Unfortunately, series resistor 29 also distorts and slows signals being passed from bus 18 to bus 19 during normal operations when bus switch transistor 22 is not isolating the busses. One of the benefits of MOS bus switches is the very low on resistance. Thus adding series resistor 29 defeats one of the primary benefits of MOS bus switches.
What is desired is a bus switch using CMOS technology. Protection from undershoot on the input is desired when the bus switch is isolating its output from its input. An active undershoot-protection circuit using CMOS transistors is desired. It is desired to maintain the low on-resistance and low capacitance of the bus switch. A more fully-isolating bus switch is desirable.